High molecular weight homopolyacrylamide and water-soluble polyacrylamide copolymers, for example acrylamide-acrylic acid copolymers or copolymers of acrylamide and cationic comonomers, are known in principle. They are used in many fields of industry, for example as thickeners, flocculants, strengtheners for paper, for oil recovery, or for mining applications. The preparation can be effected especially in the aqueous phase, for example by means of adiabatic gel polymerization.
One application of high molecular weight polyacrylamides is the use thereof for tertiary mineral oil production, specifically for what is called “polymer flooding”. This involves injecting aqueous solutions of a thickening polymer through injection wells into a mineral oil deposit, the viscosity of the aqueous polymer solution being matched to the viscosity of the mineral oil. The injection of the polymer solution forces the mineral oil in the mineral oil deposit through fine cavities in the deposit from the injection well proceeding in the direction of a further well, called the production well, and mineral oil is produced through the production well. The use of viscous aqueous polymer solutions rather than water has the advantage that the polymer solution flows through the formation much more homogeneously than water and thus more oil is mobilized than when water is used alone.
The deposit temperature of mineral oil deposits is typically above room temperature, for example 30° C. to 120° C. The aqueous polymer solution flows only very gradually through the formation, such that the residence time of the aqueous polymer solution in the formation is typically several months, and in some circumstances even years. In order to assure the mobilization of mineral oil even over this period, the viscosity of the polymer solution should not drop significantly, if at all. The viscosity of the polymer solution can decrease especially when the polymers are chemically or physically degraded.
One cause of the chemical degradation of polymers can be the presence of oxygen in the polymer solution. The polymer solutions are therefore typically produced on the oilfield by dissolving solid polymer powders in suitable dissolution systems, while attempting to exclude oxygen, for example by working under protective gas.
However, the amounts of polymer solution required for polymer flooding are high. Even for flooding of oilfields of the moderate size, it may be necessary to inject a few thousand m3 of polymer solution per day. In the case of a polymer concentration of 0.2% by weight and an injection rate of 5000 m3 per day, 10 t of solid polymer have to be dissolved per day. Under these conditions, it is difficult or at least costly and inconvenient to completely exclude oxygen. It is therefore common practice to add to the polymer solutions stabilizers or combinations of stabilizers which are supposed to prevent the degradation of the polymer by molecular oxygen or by further reactions induced by molecular oxygen. Stabilizers of this kind may be free-radical scavengers. Free-radical scavengers are supposed to react with free radicals such that the free radical is no longer able to attack and chemically degrade the polymer. Examples of stabilizers of this kind include sulfur compounds, for example 2-mercaptobenzothiazole, or sterically hindered amines. WO 2010/133258 A1 and literature cited therein give an overview of the use of various stabilizers in polymer solutions for prevention of free-radical degradation by molecular oxygen for tertiary mineral oil production.
Stabilizers of this kind can of course be added on dissolution of the solid polymers by the user. However, many users prefer to use polymer granules, polymer powders or other solid polymer preparations that already comprise stabilizers, because this reduces the cost and inconvenience for the end user.
Polyacrylamides and polyacrylamide copolymers comprising stabilizers can be prepared in various ways, for example by mixing solid polymer or a polymer gel with a likewise solid stabilizer in suitable mixers. Finally, the polymer and the stabilizer can be dissolved in water and a mixture of polymer and stabilizer can be precipitated out. The preparation can also be effected by spraying polymer gels or polymer granules with a solution of the stabilizers. This procedure has the disadvantage that the stabilizer has been applied only superficially. In the course of transportation of the polymer granules, for example in big bags, there can always be superficial abrasion. The fines fraction with a high stabilizer content typically collects at the bottom of the transport vessel, while the coarse material at the top lacks stabilizer. If only a portion of the contents is taken from the container, the amount of stabilizer in solution will possibly deviate from the desired value. All the variants mentioned have the disadvantage that it constitutes a further process step, which means additional cost and inconvenience.
JP 74027659 B describes the preparation of polyacrylamides by polymerization of acrylamide in aqueous solution. After the polymerization, the stabilizer is added to the aqueous solution, and polymer and stabilizer are finally coprecipitated out of the aqueous solution by means of suitable precipitants and dried.
The preparation of high molecular weight polyacrylamides can particularly advantageously be undertaken by means of adiabatic gel polymerization. In this case, a solution of acrylamide and optionally water-soluble copolymers is first made up in water. The concentration of the monomers may be 20 to 70% by weight. The solution is polymerized without stirring and the reactor is typically neither heated nor cooled. This gives rise to a solid polymer gel, which is dried and ground to give granules or powder.
WO 2012/069478 A1 discloses a method of tertiary mineral oil production, in which a water-soluble, hydrophobically associating copolymer at least comprising (meth)acrylamide or derivatives thereof, at least one monomer having acidic groups and at least one monomer having a terminal hydrophobic group is used. The preparation can be effected by means of gel polymerization. For this purpose, first of all, a mixture of the monomers, initiators and optionally further auxiliaries in water or an aqueous solvent mixture is provided. The polymerization can be initiated by photochemical or thermal means. The photochemical polymerization can preferably be conducted at temperatures of −5° C. to 10° C. The thermal polymerization can be initiated using a mixture of a redox initiated system and a water-soluble azo initiator at 0 to 10° C. The heat of reaction released heats the mixture, and a polymer gel is formed.
WO 2012/136613 A1 discloses a method of mineral oil production, in which permeable regions of an underground mineral oil deposit are blocked using an aqueous formulation of a hydrophobic associating copolymer, preferably with crosslinking of the polymers. The hydrophobically associating copolymer comprises at least (meth)acrylamide or derivatives thereof, a monomer having a terminal hydrophobic group and monomers having acidic groups and/or (meth)acrylic esters. The preparation can be effected by means of gel polymerization, the details disclosed in WO 2012/136613 A1 regarding the performance of gel polymerization being the same as in WO 2012/069478 A1.
DE 197 48 153 A1 discloses a process for preparing water-soluble cationic polyelectrolytes by adiabatic photopolymerization, in which a mixture of nonionic monomers and cationic monomers is undertaken in aqueous solution in the presence of a photoinitiator and of a redox system which is only capable of initiating the polymerization at temperatures≥20° C. The photopolymerization is initiated at temperatures of less than 20° C., preferably less than 10° C., and, as a result of the heating of the mixture because of heat of reaction, the redox system also initiates the polymerization later on in the preparation.
However, none of documents WO 2012/069478 A1, WO 2012/136613 A1 and DE 197 48 153 A1 discloses that stabilizers to prevent polymer degradation by molecular oxygen can be added directly to the mixture to be polymerized.
It is of course possible to mix the solid polymer gel obtained or the dried polymer with a stabilizer in a suitable mixing device, but this again would be one process step more.
There are also known techniques in which the stabilizer, in the case of a gel polymerization, is added at an early stage, before the polymerization.
U.S. Pat. No. 5,296,577 describes a process for preparing polyacrylamides by free-radical polymerization of acrylamide and optionally comonomers in an aqueous medium in the presence of azo initiators and at least 0.1% by weight of the 2-mercaptobenzothiazole stabilizer or a salt thereof at a pH of at least 6, preferably at least 7, within a temperature range from 5 to 100° C. under adiabatic conditions. Even though 2-mercaptobenzothiazole can react with free radicals, it reacts in a non-irreversible and undesirable manner with the azo initiators. In order to obtain high molecular weight polymers with high viscosity, it is desirable to initiate the polymerization at minimum temperature. U.S. Pat. No. 5,296,577, however, teaches that, in the process proposed in U.S. Pat. No. 5,296,577 at a polymerization temperature of less than 5° C., polymerization is retarded so significantly that high molecular weight polyacrylamides are no longer obtained.
DE 30 21 767 A1 describes a process for preparing high molecular weight polyacrylamides by free-radical polymerization in an aqueous medium, in which the polymerization is conducted in the presence of 2-mercaptobenzimidazole. The polymerization can be performed within the temperature range from 0° C. to 100° C. However, the examples disclose only the initiation of the polymerization at 10° C. to 30° C. and pH values ≥7.